Twenty-six patients who had cytogenetic analyses performed for myeloid malignancies at St. Vincent's Hospital over a 6-year period were found to have an inversion abnormality of chromosome 16 (25 patients) or t(16;16) (1 patient). Only 16 patients had all the features of M4Eo, while the other 10 patients had diagnoses of M2, M4, M5, RAEB, and RAEB-T; six of these had abnormal eosinophils. Thus, abnormal eosinophils were present in 22 of 26 patients (85%). Thirteen patients had additional cytogenetic abnormalities at diagnosis, the commonest being +8 in 5, del(7q) in 4, and +21 in 3. Twenty-three patients received chemotherapy and 20 (87%) achieved complete remission. The median survival of the treated group was 188 weeks with a 61% 2-year and 45% 5-year survival. No significant difference in survival was observed between those patients with a diagnosis of M4Eo and those with other diagnoses suggesting that it is the abnormality of chromosome 16 which confers an improved prognosis. Additional cytogenetic abnormalities present at diagnosis did not affect prognosis. CNS relapse was observed in only two patients (8%), thus indicating no increased incidence of this complication. This study supports the premise that a chromosome abnormality involving 16p13 and 16q22 defines a good prognosis subset of myeloid leukemia despite morphological variations.